Glutamine is an abundant non-essential amino acid that is highly involved in carbon and nitrogen transfer between different organs and tissues. Skeletal muscle is the principal source of whole body glutamine production, accounting for 50-70% of the glutamine rate of appearance (Ra). The glutamine carbon skeleton is utilized by the gut as an energy source and by the liver as a gluconeogenic precursor. Thermodynamically, glutamine is a potent gluconeogenic amino acid since its conversion to sugar phosphates is accompanied by a net gain of ATP and reducing equivalents. There is evidence that glutamine derived from peripheral tissues is a significant source of carbon for hepatic gluconeogenesis. Therefore, alterations in whole body glutamine production may have a significant effect on gluconeogenic activity and hepatic glucose metabolism.
Glutamine can be derived from both metabolic and proteolytic sources hence its Ra may be influenced by changes in peripheral metabolic activity or in whole-body protein kinetics. These could include the balance between whole-body protein anabolism and catabolism as well as the intermediary metabolic flux activities of various peripheral tissues.
Skeletal muscle has a relatively large pool of free glutamine that is in rapid turnover and is derived from both metabolic and proteolytic activities. The glutamine content of alkali-soluble muscle protein is about 4%, hence protein breakdown provides a direct source of glutamine. In addition, glutamine can be synthesized from other amino acids that are released during proteolysis, including proline, histidine, asparagine and glutamate. Of these precursors, glutamate is quantitatively the most important since its abundance in skeletal muscle protein is ˜4 fold higher than that of glutamine. Consequently, each equivalent of glutamine that is directly released by skeletal muscle proteolysis is accompanied by four of glutamate which can be potentially converted to glutamine. Glutamine can also be derived from the pool of Krebs cycle metabolites via α-ketoglutarate and glutamate. Net glutamine production from this source requires that the α-ketoglutarate that is lost from the Krebs cycle be balanced by anaplerotic inflow into the cycle. In skeletal muscle, activities of pyruvate carboxylase and malic enzyme allow the anaplerotic utilization of pyruvate. Significant levels of anaplerotic flux have been reported in rat skeletal muscle by 13C tracers.
There is a long felt medical need for a clinician to be able to identify the sources of whole body glutamine carbons for hepatic gluconeogenic activity. Critical illness is characterized by a loss of lean body mass (muscle wastage) hence there is potential for an increased generation of glutamine from protein breakdown. This setting is also characterized by hyperglycemia and elevated rates of hepatic gluconeogenesis. Given that glutamine may be a significant contributor to hepatic gluconeogenesis in healthy subjects an increased availability of hepatic glutamine during illness could contribute to elevated hepatic gluconeogenic fluxes. However, to date, it has not been possible to detect to what extent whole-body protein breakdown and cataplerotic fluxes contribute to the hepatic glutamine pool. Current methods can estimate the contribution of glutamine released directly from protein breakdown to whole body glutamine Ra. However, to the extent that glutamine is synthesized from other amino acids released during protein breakdown (notably glutamate), the prior art methods substantially underestimate the overall contribution of protein degradation to the hepatic glutamine pool and are not useful for early detection of muscle wastage.
It is therefore an object of the present invention to obviate the disadvantages associated with the prior art and provide improved methods for distinguishing between the origins of the carbon skeletons of hepatic glutamine.